Vehicle suspension systems typically include a spring component or components and a damping component or components. Frequently those discrete components are separately mounted on a vehicle. Traditionally, mechanical springs, such as metal leaf or helical springs, have been used in conjunction with some type of viscous fluid based damping mechanism mounted functionally in parallel. More recently, compressed gas acting over a piston area has replaced mechanical springs as the spring component in some contemporary suspension systems. While compressed gas springs are usually lighter and more compact than their mechanical counterparts, the compression and expansion curve, and corresponding spring rate, is not linear and becomes particularly exponential beyond a mid range of gas compression. FIG. 1 is a graph showing a gas compression curve having such an exponential spring rate in an air shock based upon force and travel.
As can be seen in the Figure, the force (corresponding to pressure acting on a given piston area) versus the linear travel or displacement of the air spring is not linear. While the curve approximates linearity during an initial portion of travel, the last portion of travel is exponential. At higher travel values, the rate of increase of the force (pressure) for incrementally further travel is very large and the shock absorber is therefore increasingly more rigid in the last third of its stroke.
Sequentially activated gas spring chambers have been devised in an attempt to derive a more useable gas spring rate over a greater range of suspension travel. One such device is the subject of U.S. Pat. No. 4,915,364 which patent is incorporated herein, in its entirety, by reference. That patent describes some features of a gas spring for use in heavy vehicle applications (e.g. truck rear axles) and a Figure from that patent is included as FIG. 2. As is shown in the Figure, the spring includes a center shaft 44 having a mushroom valve 46 at an upper end that obstructs a gas flow pathway into a secondary gas chamber 54. As the gas spring operates, the shaft 44 in the center of the spring contacts an upper portion of a movable body, thereby opening the mushroom valve 46 and permitting the available gas chamber volume to be increased by the volume of the second chamber 54. A gas spring curve associated with the device of FIG. 2 is shown in FIG. 3. The point Q shows that point where the mushroom valve opens and combines the first and second gas chambers.
While the '364 reference teaches some basics of an air spring with dual chambers, it would be ineffective with many vehicle types. For example, the shaft 44 responsible for operating the mushroom valve 46 relies on gravity to close the valve as the gas spring rebounds. In many vehicle types, such a design is would be unworkable since vehicles often operate on uneven surfaces where the gravitational forces cannot be relied upon to consistently open or close a valve (and may cause arbitrary opening and closing with no functional benefit). Further, many vehicles include springs having axes mounted horizontally (or other non-vertical), upside down or at other angles inconsistent with gravity based operational features. Motorcycles, all-terrain-vehicles (ATVs) and bicycles are just some examples of vehicles that are designed for “off road” operation and require that components operate regardless of gravitational forces. Moreover, the design illustrated in the '364 patent is limited to use with simple air-type springs and not with more sophisticated mechanisms that include fluidly isolated dampers. The '364 patent suggests that the second gas chamber is prefilled to a predetermined pressure matching that of the lower chamber just prior to opening of the mushroom. However, there is no teaching as to how such a filling might take place, i.e., the pre-filling of essentially two separate chambers.
Accordingly, there is a need for a damped shock absorber that uses a multiple volume gas spring under a variety of loads and/or under a variety of travel settings. There is a further need for methods and apparatus to selectively direct gas pressure among and between a plurality of gas volumes and one that permits integration into a complex shock absorber that includes adjustable spring and dampening devices.